The present application claims priority to Japanese Applications Nos. P2000-128999 filed Apr. 25, 2000 and P2000-129000 filed Apr. 25, 2000, which applications are incorporated herein by reference to the extent permitted by law.
This invention relates to a positive electrode active material, capable of reversibly doping/undoping lithium, and to a non-aqueous electrolyte cell which uses this positive electrode active material.
Recently, with rapid progress in a variety of electronic equipment, investigations into a re-chargeable secondary cell, as a cell that can be used conveniently economically for prolonged time, are proceeding briskly. Representatives of the secondary cells are a lead accumulator, an alkali accumulator and a lithium secondary cell.
Among these secondary cells, a lithium secondary cell has various advantages, such as high output or high energy density. The lithium secondary cell is made up of a positive electrode and a negative electrode, each having an active material capable of reversibly doping/undoping lithium, and a non-aqueous electrolyte.
Among known positive electrode active materials of the lithium secondary cell, there are a metal oxide, a metal sulfide and a polymer. For example, there are known lithium-free compounds, such as, for example, TiS2, MoS2, MbSe2 or V2O5, and lithium compound oxides, such as LiMO2, where M is Co, Ni, Mn or Fe, or LiMn2O4.
As the positive electrode active material, having the potential of 4V with respect to lithium, LiCoO2 is being put to extensive use. This LiCoO2 is an ideal positive electrode in many respects in that it has a high energy density and a high voltage.
However, Co is a rare resource localized on the earth and hence it is expensive. Moreover, it cannot be furnished in stability without considerable difficulties. So, a demand is raised towards developing a positive electrode material which is based on inexpensive Ni or Mn and which is present in abundance as a resource.
The positive electrode containing LiNiO2 has a large theoretical capacity and a high discharging potential. However, it has such a defect that, with the progress in the charging/discharging cycle, the crystal structure of LiNiO2 is collapsed to lower the discharging capacity as well as the thermal stability.
As a Mn-based positive electrode material, LiMn2O4 having a positive spinel structure and a spatial group Fd3m has been proposed. This LiMn2O4 has a high potential of the 4V-grade potential with respect to lithium, which is equivalent to LiCoO2. Moreover, LiMn2O4 is easy to synthesize and high in cell capacity so that it is a highly promising material and is being put to practical use.
However, the cell formed using LiMn2O4 has a drawback that it undergoes serious deterioratiuon in capacity on storage at elevated temperatures, while it is insufficient in stability and cyclic characteristics, with Mn being dissolved in an electrolytic solution.
So, it has been proposed in Japanese Laying-Open Patent H-9-134724 to use a phosphoric acid compound of a transition metal M having an olivinic structure as a positive electrode of the lithium ion cell, where M is Fe, Mn, Co or Ni. It has also been proposed in Japanese Laying-Open Patent H-9-171827 to use e.g., LiFePO4, among the phosphoric acid compounds of the transition metal M having an olivinic structure.
It is noted that LiFePO4 has a volumetric density as high as 3.6 g/cm3 and develops a high potential of 3.4 V, with the theoretical capacity being as high as 170 mAh/g. Moreover, in the initial state, LiFePO4 contains electrochemically dopable Li at a rate of one Li atom per Fe atom, and hence is a promising material as a positive electrode active material of the lithium ion cell.
However, as reported in the above patent publication, a real capacity only on the order of 60 to 70 mAh/g has been realized in an actual cell which uses LiFePO4 as a positive electrode active material. Although the real capacity on the order of 120 mAh/g has been subsequently reported in the Journal of the Electrochemical Society, 144,1188 (1997), this capacity cannot be said to be sufficient in consideratiuon that the theoretical capacity is 170 mAh/g. There is also a problem that the discharging voltage of LiFePO4 is 3.4V which is lower than that of the positive electrode active material used in the current lithium ion cell.
So, it has been proposed to use LiMnPO4, as a phosphoric acid having an olivinic structure, comprised mainly of Mn, which is an element having a redox potential highrer than that of Fe, as the positive electrode of the lithium ion cell.
However, in the Mn-based routine phosphoric acid compound, comprised basically of LiMnPO4, it is difficult to yield Mn by the redox reaction. It is reported in the aforementioned Journal of the Electrochemical Society, 144,1188 (1997) that, of the Mn-based phosphric compounds of the olivinic structure, only LiMnxFe1-xPO4, in which Fe is substituted for part of Mn, is the sole phosphoric compound in which Mn can be generated by a redox reaction.
In the above treatise, there is a report that an actual cell constructed using LiMnxFe1-xPO4 as a positive electrode active material can develop a real capacity of the order of 80 mAh/g. However, this capacity may not be said to be sufficient in consideratiion that the theoretical capacity is 170 mAh/g.
In the above treatise, there is a report that, in an actual cell which uses LiMnxFe1-xPO4 as a positive electrode active material, the capacity is decreased when the proportion y of Mn exceeds 0.5. That is, according to the teaching in the above treatise, if the proportion Mn in LiMnxFe1-xPO4 is increased, the capacity is decreased, even though the high voltage is achieved, and hence the compound is not suitable as a material for practical use. If conversely the proportion of Mn in LiMnxFe1-xPO4 is lowered for realizing a high capacity, the proportion of Mn as a main reaction partner in the redox is lowered, with the result that the high redox potential proper to Mn cannot be sufficiently achieved. In addition, if the discharging voltage is lowered, the cell produced ceases to be compatible with the currently used lithium ion cell.
So, it is extremely difficult with LiMnxFe1-xPO4 to realize high capacity and high discharge voltage simultaneously.
On the other hand, in the Mn-based phosphoric acid compound, having the olivinic structure, M has a high redox potential and hence the compound is expected to manifest excellent properties. However, only a few of the compounds may be used in a cell. Thus, a demand is raised towards development of the phosphoric acid compound having the olivinic structure.
It is therefore an object of the present invention to provide a positive electrode active material capable of realizing a high discharging capacity without lowering the capacity in order to manifest superior charging/discharging characteristics and a non-aqueous electrolyte cell which uses the positive electrode active material.
It is another object of the present invention to provide a positive electrode active material in which Mn generation by redox, not possible so far, is realized without lowering the capacity, and which exhibits a high discharging voltage and superior charging/discharging characteristics.
It is yet another object of the present invention to provide a non-aqueous electrolyte cell which uses such positive electrode active material.
In one aspect, the present invention provides a positive electrode active material containing a compound represented by the general formula LixMnyFe1-yPO4, where 0 less than xxe2x89xa62 and 0.5 less than y less than 0.95.
In this positive electrode active material, Fe is substituted for a portion of Mn of LixMnyFe1-yPO4. Since this Fe is able to dilute the Yarn-Teller effect ascribable to Mn3+, it is possible to suppress distortion of the crystal structure of LixMnyFe1-yPO4. Since the proportion y of Mn is in a range of 0.5 less than y less than 0.95, a high discharge voltage can be achieved without lowering the cell capacity.
In another aspect, the present invention provides a positive electrode active material containing a compound represented by the general formula LixMnyFezA1-(y+z)PO4, where 0 less than xxe2x89xa62, 0.5 less than y less than 0.95, 0.5 less than y+z less than 1, and A is at least one metal element selected from Ti and Ag.
In this positive electrode active material, Fe and the metal element A are substituted for a portion of Mn in LixMnyFezA1-(y+z)PO4. Since this Fe and the metal element A are able to dilute the Yarn-Teller effect ascribable to Mn3+, it is possible to suppress distortion of the crystal structure of LixMnyFe1-yPO4. Since the proportion y of Mn is in a range of 0.5 less than y less than 0.95, a high discharge voltage can be achieved without lowering the capacity.
In another aspect, the present invention provides a non-aqueous electrolyte cell including a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material and an electrolyte interposed between the positive and negative electrodes, wherein the positive electrode active material contains a compound represented by the general formula LixMnyFe1-yPO4 where 0 less than xxe2x89xa62 and 0.5 less than y less than 0.95.
In the above-described non-aqueous electrolyte cell, the Yarn-Teller effect ascribable to Mn3+ is diluted to enable Mn to be yielded by the redox reaction. So, the non-aqueous electrolyte cell employing this positive electrode active material exhibits superior charging/discharging characteristics.
In another aspect, the present invention provides a non-aqueous electrolyte cell including a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material and an electrolyte interposed between the positive and negative electrodes, wherein the positive electrode active material contains a compound represented by the general formula LixMnyFezA1-(y+z)PO4 where 0 less than xxe2x89xa62, 0.5 less than y less than 0.95 and 0.5 less than y+z less than 1 and wherein A is at least one metal element selected from Ti and Mg.
With the above-described non-aqueous electrolyte cell, the Yarn-Teller effect ascribable to Mn3+ is diluted to enable Mn to be yielded by the redox reaction. Moreover, since the proportion y of Mn in LixMnyFezA1-(y+z)PO4 is in a range of 0.5 less than y less than 0.95, a high discharge voltage can be achieved without lowering the cell capacity. Therefore, the non-aqueous electrolyte cell employing this positive electrode active material exhibits superior charging/discharging characteristics.
The present inventors also have conducted eager searches towards accomplishing the above object, and have found that Mn redox is difficult because the Yarn-Teller effect is produced due to Mn3+ generated in the charged state to cause distortion of the crystal structure of the phosphoric acid compound having the olivinic structure. This finding has led to the concept of a positive electrode active material according to the present invention.
In another aspect, the present invention provides a positive electrode active material containing a compound represented by the general formula LixMnyB1-yPO4, where 0 less than xxe2x89xa62 and 0 less than y less than 1 and wherein B is a metal element selected from among Ti, Zn, Mg and Co.
In this positive electrode active material, one metal element selected from among Ti, Zn, Mg and Co is substituted for a portion of Mn of LixMnyB1-yPO4 as a phosphoric acid compound having the olivinic structure. Since this metal element is able to dilute the Yarn-Teller effect ascribable to Mn3+, distortion of the crystal structure of LixMnyB1-yPO4 can be prevented from occurring.
In another aspect, the present invention provides a positive electrode active material containing a compound represented by the general formula LixMnyB1-yPO4, where 0 less than xxe2x89xa62 and 0 less than y less than 1 and wherein B denotes plural metal elements selected from among Ti, Fe, Zn, Mg and Co.
In this positive electrode active material, plural metal elements selected from among Ti, Fe, Zn, Mg and Co are substituted for a portion of Mn of LixMnyB1-yPO4 which is a phosphoric acid compound having the olivinic structure. Since this metal element B is able to dilute the Yarn-Teller effect ascribable to Mn3+, distortion of the crystal structure of LixMnyB1-yPO4 can be prevented from occurring.
In another aspect, the present invention provides a non-aqueous electrolyte cell including a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material and an electrolyte interposed between the positive and negative electrodes, wherein the positive electrode active material contains a compound represented by the general formula LixMnyB1-yPO4 where 0 less than xxe2x89xa62 and 0 less than y less than 1 and wherein B denotes one metal element selected from among Ti, Zn, Mg and Co.
This non-aqueous electrolyte cell contains LixMnyB1-yPO4, as a positive electrode active material, in which a metal element B selected from among Ti, Zn, Mg and Co is substituted for a portion of Mn. Since the metal element B in the LixMnyB1-yPO4, used as positive electrode active material, is able to dilute the Yarn-Teller effect ascribable to Mn3+, distortion of the crystal structure of LixMnyB1-yPO4 can be prevented from occurring, thus realizing a non-aqueous electrolyte cell having a high discharge capacity and superior charging/discharging characteristics.
In another aspect, the present invention provides a non-aqueous electrolyte cell including a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material and an electrolyte interposed between the positive and negative electrodes, wherein the positive electrode active material contains a compound represented by the general formula LixMnyB1-yPO4 where 0 less than xxe2x89xa62 and 0 less than y less than 1 and wherein B denotes plural metal elements selected from among Ti, Fe, Zn, Mg and Co.
This non-aqueous electrolyte cell contains LixMnyB1-yPO4, as a positive electrode active material, in which plural metal elements B selected from among Ti, Fe, Zn, Mg and Co is substituted for a portion of Mn. Since the metal element B in the LixMnyB1-yPO4, used as positive electrode active material, is able to dilute the Yarn-Teller effect ascribable to Mn3+, distortion of the crystal structure of LixMnyB1-yPO4 can be prevented from occurring. Thus, with LixMnyB1-yPO4, redox generation of Mn is possible, so that a non-aqueous electrolyte cell having a high discharge capacity and superior charging/discharging characteristics may be produced.
According to the present invention, part of Mn of LixMnyFe1-yPO4, used as a positive electrode active material, is replaced by Fe. Since this Fe is able to dilute the Yarn-Teller effect ascribable to Mn3+, distortion of the crystal structure of LixMnyFe1-yPO4 may be prevented from occurring. Moreover, since the proportion y of Mn is such that 0.5 less than y less than 0.95, the range of the high discharge voltage area in the vicinity of 4V can be enlarged without decreasing the capacity. Thus, Mn can be produced by a redox reaction to allow to furnish a positive electrode active material capable of realizing a high capacity and a high discharge capacity.
Moreover, according to the present invention, part of Mn of LixMnyFe1-(y+z)PO4, used as a positive electrode active material, is replaced by Fe and a metal element A. Since this Fe and the metal element A are able to dilute the Yarn-Teller effect ascribable to Mn3+, distortion of the crystal structure of LixMnyFe1-(y+z)PO4 may be prevented from occurring. Moreover, since the proportion y of Mn is set so that 0.5 less than y less than 0.95, the range of the high discharge voltage area in the vicinity of 4V can be enlarged without decreasing the capacity. Thus, Mn can be produced by a redox reaction to allow to furnish a positive electrode active material capable of realizing a high capacity and a high discharge capacity.
According to the present invention, LixMnyFe1-yPO4, in which Mn can be produced on redox and which achieves a high capacity and a high discharge voltage, is used as the positive electrode active material of the non-aqueous electrolyte cell. Thus, a non-aqueous electrolyte cell may be furnished having superior charging/discharging characteristics and which may be made compatible with the customary lithium cell.
Moreover, according to the present invention, LixMnyFe1-(y+z)PO4, in which Mn can be produced on redox and which achieves a high capacity and a high discharge voltage, is used as the positive electrode active material of the non-aqueous electrolyte cell. Thus, a non-aqueous electrolyte cell may be furnished having superior charging/discharging characteristics and which may be made compatible with the customary lithium cell.
The positive electrode active material according to the present invention contains Mn-based LixMnyA1-yPO4 of the olivinic structure, in which a metal element selected from among Ti, Zn, Mg and Co is substituted for part of Mn. In this LixMnyA1-yPO4, a metal element selected from among Ti, Zn, Mg and Co is substituted for part of Mn, or plural metal elements selected from among Ti, Fe, Zn, Mg and Co are substituted for part of Mn. Since the metal element A is able to dilute the Yarn-Teller effect ascribable to Mn3+, it is possible to suppress distortion of the crystal structure of LixMnyA1-yPO4. Thus, according to the present invention, a positive electrode active material may be furnished in which Mn generation on redox, so far retained to be difficult, can be realized to assure a high discharge voltage and superior charging/discharging characteristics.
The non-aqueous electrolyte cell according to the present invention uses LixMnyA1-yPO4, capable of generating Mn on redox, is used as positive electrode active material, thus realizing a non-aqueous electrolyte cell having a high discharge voltage and superior charging/discharging characteristics.